Superhydrophobic surfaces exhibit a high degree of water-repellency. Water forms almost spherical droplets on them that readily roll away. This phenomenon is known as the “lotus effect”, which has attracted tremendous attention in the past few years due to the large number of potential applications. A generally accepted definition for a superhydrophobic surface is a surface on which the contact angle of a water droplet exceeds 150° and the roll-off angle (i.e., the minimum tilt angle of the surface at which a droplet starts rolling on it) is less than 10°. The superhydrophobic effect, namely superhydrophobicity, results from a synergism of proper surface roughness and low surface energy. The roughness is usually generated by introducing a porous surface layer having micro- and/or nano-structures. When covered with water, the porous surface layer can entrap air, resulting in a composite interface where water only partially wets the solid substrate. Besides the proper surface roughness, an intrinsically hydrophobic surface layer is essential to obtain the superhydrophobicity.
Coating a non-superhydrophobic surface is a convenient way to create a superhydrophobic surface. To date, many ways to produce superhydrophobic coatings have been reported. However, very few examples are available for real applications, for they are usually not durable. This seriously limits the practical applications of superhydrophobic surfaces. When exposed to an outdoor environment, low surface energy materials of the superhydrophobic coatings can be decomposed by sunlight, and micro/nanostructures of the surfaces are easily destroyed under severe weather conditions, thus leading to permanent destruction of the superhydrophobicity. In contrast to artificial superhydrophobic coatings, natural plant leaves like lotus leaves can maintain their superhydrophobicity by repairing the damaged surface layer with a continuously-secreting hydrophobic epicuticular wax (C. Neinhuis, K. Koch and W. Barthlott, Planta, 213, 427 (2001)). It is believed that a self-healing function similar to that of the plant leaves is needed for a durable superhydrophobic coating.